In this type of fuel cell system, it has been requested that the reforming water be supplied to the evaporation unit with high accuracy to obtain a target amount of water vapor. Various techniques have been disclosed for this reason.
JP 2004-288505 A discloses a reforming device for a fuel cell that has a water tank configured to store the reforming water, and a water pump configured to supply the reforming water in the water tank to the evaporation unit. According to this, by detecting a change in water level in the water tank configured to store the reforming water, and by performing a feedback control of the change in water level to the water pump, the flow rate of the reforming water stored in the water tank is adjusted. This allows the flow rate of reforming water supplied to the reforming unit to be adjusted to a desired amount of water.
JP 2012-133915 A discloses a fuel cell system that avoids water wetting of a reforming catalyst (because introduction of the reforming water is too early) and caulking (because introduction of the reforming water is too late), by keeping the reforming water level in a place immediately before an inlet of the evaporation unit of the reformer at the time of the system startup, and by suppressing the variation of the timing of introducing the reforming water after igniting the reformer. In general, a water flow meter is used to detect whether the flow rate of the reforming water is normal, but since the flow rate is small, measurement accuracy of the flow rate is low. Moreover, the flow meter is very expensive, and there is also a problem in the durability. Therefore, in the above-described system, a water sensor configured to detect the presence of water has been used while abolishing the flow meter, and it has been considered that water supply accuracy of the reforming water to the evaporation unit is secured by satisfying the following items.
water is present in the water tank (detected by the water level sensor)
the water pump is normally rotating (detected by the pump rotational speed output)
water is present in the water supply passage (detected by the water sensor)
However, when the water pump is locked and enters a step-out (the water pump is locked by biting of foreign matter or the like) state, there is an output of the pump rotational speed, but the water pump is in a state of not being able to discharge the water. In this case, since the water pump cannot discharge the reforming water in the water tank to the evaporation unit side, the reforming water is not transmitted to the evaporation unit. However, even if the water does not flow, as long as the water is present in a sensing unit of the water sensor, there is a possibility that the water sensor erroneously detects the water presence state, that is, a state in which the water is flowing. Thus, in the state in which the reforming water is not supplied to the evaporation unit, a power generation operation of the fuel cell system may be continued. In this case, there is a concern about caulking (generation of hydrocarbon components of the raw material gas) of the reforming catalyst in the reforming unit, and damage to the fuel cell.